I have noticed a problem on several of my Sony Beta VCR's. Most of these are the BetaScan top loading units with mechanical controls, a row of varactor channel buttons on top and the "fast play" feature (can't remember the model number) but it has happened on a later front loader too.
Audio is either weak or mixed with fuzzy noise. Occasionally one of the machines will have a whining whistling sound in the audio which goes away if it's tapped.

Could this be a bad cap or other electronic problem? Cleaning does not seem to help. Video is good...just the audio is poor.

If tapping the unit improves the audio, then it's probably a loose connection inside somewhere.

SL-?? Model numbers might help - most of my literature has been marked up by techs with the symptom/cure. I do know of some Beta machines that had poor playback audio - they had bad audio heads - the humidity would get them. Here on the Atlantic coast, it's a problem that's quite common.

Cheers,

There were three models with that basic design. SL-5400, SL-5600, and SL-5800.

Thank you...next time I am at the machines I will make note of the model numbers.

On the older SL-8200 and 8600 era machines, there's a series of switches that are the 20-ish pin connection slide type similar to what audio recorders used for the record changeover switch. These mechanical critters tend to get dirty and noisy, which would explain the change in whistling when tapping. Deoxit does wonders. I'd look for it on the main motherboard on the bottom. You'll have to remove the entire bottom to access. Often two or more of the connections act as convenient hinges so be sure not to unscrew them by mistake.